1). Field of the Invention
This invention relates generally to a method of substrate processing and, more specifically, to a method of photoresist exposure.
2). Discussion of Related Art
Integrated circuits are manufactured on silicon and other semiconductor substrates often in the form of wafers. A wafer is obtained by drawing a silicon ingot from liquid silicon and then sawing the ingot into many wafers. Various layers are then formed on the wafer followed by a photoresist layer. A mask having openings therein is then located over the photoresist layer and a light source is located over the mask so that light shines through the openings onto selected areas of the photoresist layer. These selected areas change chemical composition so that they are subsequently selectively etched over areas of the photoresist layer that are not exposed to the light. The photoresist layer is then used as a mask to etch layers below the photoresist layer.
A cutting edge of a wire used for sawing the wafers makes a series of alternating raised and recessed formations on each wafer. The formations extend in a direction of the cutting edge and are spaced from one another in a direction in which the saw progresses through the wafer. These formations are reflected on the photoresist layer. The light source emanates a bundle of light which is wider than it is long, and, along its width, crosses over many of the formations at a given moment. The light has a uniform focal length across its width, usually about halfway between peaks of the raised formations and lower points of the recessed formations. The light is thus out of focus at the peaks and lower points, resulting in inaccurate exposure of the photoresist layer. Inaccurate exposure of the photoresist layer results in inaccurate dimensions of formations etched in layers below the photoresist layer.